The current trend of tissue engineering technology is toward the development of biomaterials for repairing tissue defects or to enhance fixation of implants to the host tissue. Basic requirements include a scaffold-implant conductive to cell attachment and maintenance of cell function, together with a rich source of progenitor cells. Biomaterials in combination with cells from ex-vivo cultures will not only accelerate the tissue healing, but also increase the biocompatibility of scaffold-implants to shorten the hospitalization, and improved long-term function of the devices. In particular, patients with large defects, impaired bone healing and cancer disease in the region of repair shall benefit from this new technology. One regenerative tissue engineering approach involves a process known as “tissue induction”, whereby a two or three-dimensional polymer or mineral scaffold-implant without cells is implanted into a patient. With tissue induction, tissue generation occurs through ingrowth of surrounding tissue into the scaffold-implant.
Another approach to tissue generation, known as “cell transplantation”, involves seeding a scaffold-implant with cells, cytokines, and other growth-related molecules, then culturing and implanting the scaffold-implant into the subject to induce the growth of new tissue. Cultured cells are infused in a biodegradable or non-biodegradable scaffold-implant, which may be placed in a bioreactor in-vitro to allow the cells to proliferate before the cells containing scaffold-implant is implanted in the patient. Alternatively, the cell-seeded scaffold-implant may be directly implanted, in which case the patient's body acts as an in-vivo bioreactor. Once implanted, in-vivo cellular proliferation and, in the case of absorbable scaffold-implants, concomitant bio-absorption of the scaffold-implant, proceeds.
In both types of tissue engineering, i.e., tissue induction and cell transplantation, the scaffold-implant, whether or not bio-absorbable, must be biocompatible, such that it does not invoke an adverse immune response from, or result in toxicity to, the patient.
Several types of materials have been investigated for use as seeding scaffold-implants, including metals, ceramics, polymers, and polymer-coated metals and ceramics. Existing scaffold-implants may be manufactured by solvent casting, shaping sections with machining, 3D printing, or molded collagen/cell constructs. While the aforementioned scaffold-implant materials are primarily for industrial applications, the fabrication of hydroxyapatite scaffold-implants using selective laser sintering and polymer-coated calcium phosphate powder, have been investigated. Additional post-processing, such as high temperature heating which burns out the binder, and then higher temperature sintering which fuses the powder together, is required to strengthen the scaffold-implant.
Whichever type of scaffold-implant is selected, a purpose of the scaffold-implant is to support cells, which, after being seeded into the device, cling to the interstices of the scaffold-implant, replicate, produce their own extra-cellular matrices, and organize into the target tissue. For example, in the case of bone regeneration, the optimal pore size for maximum tissue growth ranges from 200-400 mircons (μm). Therefore, the material or materials used for fabricating the scaffold-implant should have this pore size (200-400 μm) and have sufficient rigidity and biomechanical properties to support loads that are used for generating bone tissue.
Scaffold-implants fabricated from a material such as hydroxyapatite, which is useful for supporting bone cells, are too brittle and non-pliable to act as scaffolding for muscle or tendons. Many of the polymers, and the polymer-coated metals and ceramics present a challenge to seeding cells in three-dimensional scaffolds. None of the known scaffold-implant materials allow growth of cells to a depth of greater than about 250 μm, which is a generally accepted practical limit on the depth to which cells and nutrients can diffuse into scaffold-implants having the desired porosities. Even if cells could be made to diffuse to greater depths, it is generally believed that to support cell growth and avoid or at least curtail apoptosis at these depths, the scaffold-implant must also support some form of vasculature to promote angiogenesis; none of the earlier mentioned scaffold-implant fabrication methods, however, allow for incorporation of blood vessels.
Porous, three-dimensional metallic structures have recently been developed for potential application in reconstructive orthopaedics and other surgical disciplines. Such structures are described in U.S. Pat. No. 5,282,861 entitled “Open Cell Tantalum Structures For Cancellous Bone Implants And Cell And Tissue Receptors” issued to Kaplan; U.S. Pat. No. 5,443,515 entitled “Vertebral Body Prosthetic Implant With Slidably Positionable Stabilizing Member” issued to Cohen et al.; U.S. Pat. No. 5,755,809 entitled “Femoral Head. Core Channel Filling Prothesis” issued to Cohen et al.; U.S. Pat. No. 6,063,442 entitled “Bonding Of Porous Materials To Other Materials Utilizing Chemical Vapor Deposition” issued to Cohen et al.; and U.S. Pat. No. 6,087,553 entitled “Implantable Metallic Open-Celled Lattice/Polyethylene Composite Material And Devices” issued to Cohen et al., the disclosures of which are incorporated herein by reference. The porous, three-dimensional metallic structure is a bio-compatible material having a three-dimensional network of continuously interconnected channels or pores which define a three-dimensional porosity, i.e., volume porosity, ranging from 50 to 90% (higher than all other known implant materials). This high bulk volume porosity readily facilitates nutrient diffusion and media circulation.
The porous, three-dimensional metallic structures may be fabricated using a vapor deposition/infiltration process wherein tantalum, which has a long history of medical uses, or other bio-compatible metal or material is vaporized at high temperature and precipitated as a thin layer onto a carbon lattice. The coating of tantalum or other metal enhances or improves the strength or mechanical characteristics of the carbon lattice.
As a scaffold for “bone induction,” preliminary animal studies with transcortical (bone conduction) porous, three-dimensional metallic structures have been shown to support rapid and extensive bone ingrowth. For example, tissue response to porous tantalum acetabular cups indicates that the porous tantalum material is effective for biologic fixation. The biomechanical property of the porous tantalum biomaterial is sufficient to withstand physiological load for specific applications, such as an acetabular cup, a spinal fusion, and a vertebral body replacement in fractures or in metastatic cancer disease. As a scaffold for “cell transplantation,” porous, three-dimensional metallic structures can extend culturing of multipotent hematopoietic progenitors without cytokine augmentation and enhance maintenance and retroviral transduction of primitive hematopoietic progenitor cells.
Hyaluronan possesses biochemical and physical properties suitable to perform an important role in the early events of osteogenesis as well as in many other tissues. A low-molecular weight hyaluronan fully expresses the in-vitro steogenic potential of mesenchymal cells through the subsequent proliferation and differentiation of osteoprogenitor cells using proper conditions. Locally applied high-molecular hyaluronan of MW 1900 kDa also has been shown to be capable of accelerating new bone formation through mesenchymal cell differentiation in femur wounds. Hyaluronan at a low concentration (0.5 mg/mL) has been shown to increase the development of porcine embryos in culture.